Future wireless networks most probably will comprise a large portion of multi-access networks. A typical example is a network composed of a wide area coverage system providing moderate bitrate services to mobile users, complemented by a local area coverage system providing high bitrate services to users in hotspots. In such a system, both the user behaviour and the system characteristics call for better user quality, e.g. higher bitrates, which is being offered in the hotspots.
A combined system with heterogeneous, location-dependent, quality requirements thus appears. The overall capacity C can then be defined as the largest number of users U for which both the wide area (Qwide_min) and local area (Qlocal_min) quality requirements are fulfilled:C=max{U:Qwide(Uwide)>=Qwide_min&Qlocal(Ulocal)>=Qlocal_min}A general goal is to keep the users satisfied. Satisfaction of the user or experienced communication utility can therefore serve as an optimisation parameter. As the potential experienced communication utility generated by the system depends on the overall capacity, the question emerges of how to maximise this overall capacity.
However, there is generally no linear relationship between the experienced utility for the user and maximisation of the overall capacity. Instead, almost any relation between user experienced utility and capacity may exist. Taking this into account, the utility maximisation problem may be reformulated as:
            max      ⁢                        ∑                      i            =            1                    m                ⁢                  R                      wide            ,            i                                +                  ∑                  j          =          1                n            ⁢                        R                      local            ,            j                          :                              Q                          wide              ,              i                                >                      Q            wide_min                                ,            Q              local        ,        j              >          Q      local_min      
where Rwide,i, Rlocal,j are the experienced utility for each user i, j in wide area and local area, respectively.
Also for the operators, experienced utility is of interest. WCDMA-GSM is an existing multi-service multi-access system. There can be different pricing policies and revenues for different services; e.g. voice, video-telephony and best effort data. The higher the experienced utility is, the higher price the users are prepared to pay. The service price and revenue may also differ between users as well. Different service sets and service quality may be offered on the different accesses, e.g. video telephony is offered only on WCDMA and a lower maximum best-effort packet data rate is offered on GSM.
One possible access selection principle is that there is a preferred access for each mobile. The preferred access is selected if there is coverage and capacity, which means that the service can be offered. If there is no capacity to offer the service, admission is rejected and an attempt to establish the service on the other access is initiated. This is referred to as directed retry. Service-based access selection has also been proposed for WCDMA-GSM [3]. In this case, the user is allocated to the access technology where its currently requested service is expected to be most efficiently supported. An example is to allocate voice calls to GSM and data sessions to WCDMA. It has also been proposed to measure the actual radio resource consumption of users, e.g. in terms of required power level, and allocate users based on this.
Combinations of cellular systems like GSM and WCDMA and systems of WLAN-type, e.g. IEEE 802.11, are other examples of multi-access systems. For such systems access selection based on estimated signal strength has been proposed, see e.g. [1-2].
In U.S. Pat. No. 6,163,694, here denoted as reference [4], a method for cell selection in a hierarchic cellular telephone system is disclosed. A cell selection in a lowest possible hierarchic level is desired, where a downlink signal strength exceeds a certain threshold. Within a hierarchic level, the cell with the highest measured downlink signal strength is selected.
If one of the access networks generally provides better conditions, typically the local area coverage system, one approach is to allocate as many users as possible to the local area system until it reaches its capacity limit. Then users are allocated to the wide area system. This can easily be realised by letting users first try to access the local area system, and if this fails redirect their access attempts to the wide area system. This is a directed retry approach having a fixed preferred access for all users.
A problem with this approach is that the system relatively frequently reaches a situation, where all access attempts have to go through the directed retry mechanism. Such mechanisms involve large control signalling efforts and become a non-negligible load on the communications system.